Export new generic size_sum()

[krlmlr]

Closes #173.

[DavisVaughan]

I think this looks good! Here is an example of how I could use this to solve #173, using either a list or a list-of. I think my suggested tests to add would catch both of these cases

library(pillar)
library(tibble)
library(vctrs)

tbl1 <- new_tibble(
  list(
    type = c("warning", "error"), 
    message = c("foo", "bar")
  ), 
  nrow = 2L, 
  class = "tbl_notes"
)

tbl2 <- new_tibble(
  list(
    type = c("warning", "error", "error"), 
    message = c("foo", "bar", "baz")
  ), 
  nrow = 3L, 
  class = "tbl_notes"
)

vec_ptype_abbr.tbl_notes <- function(x, ...) {
  "tbl-notes"
}
type_sum.tbl_notes <- function(x) {
  "tbl-notes"
}
size_sum.tbl_notes <- function(x) {
  w <- sum(x$type == "warning")
  e <- sum(x$type == "error")
  paste0("[", w, " W / ", e, " E]")
}

col1 <- list(tbl1, tbl2)
col2 <- list_of(tbl1, tbl2)

tibble(col1 = col1, col2 = col2)
#> # A tibble: 2 x 2
#>   col1                                 col2
#>   <list>                  <list<tbl-notes>>
#> 1 <tbl-notes [1 W / 1 E]>       [1 W / 1 E]
#> 2 <tbl-notes [1 W / 2 E]>       [1 W / 2 E]

[DavisVaughan]

Potential issue when there is no size and you put it in a list-of

library(pillar)
library(tibble)
library(vctrs)

x <- structure(1, class = "foo")
lst <- list_of(x, x)

size_sum.foo <- function(x) {
  ""
}

tibble(lst = lst)
#> # A tibble: 2 x 1
#>           lst
#>   <list<foo>>
#> 1            
#> 2

^{Created on 2021-01-28 by the reprex package (v0.3.0.9001)}

[krlmlr]

Should we use a placeholder in pillar_shaft.vctrs_list_of() if the size is empty?

[krlmlr]

Maybe size_sum() should be responsible only for the inner contents, and pillar still provides the brackets? (Or only adds brackets in the list-of case if it's empty?)